The goal of this study is to determine the structural bases of cell interactions. Haploid and diploid cells of Myxomycetes will be employed as models to investigate the phenomena of contact inhibition, phagocytosis, sexual and somatic fusions, and abnormal multiple cell fusions. Loss of contact inhibition and the tendency toward multiple cell fusions characterize mammalian cells which have undergone malignant transformation. This study is proposed in the hope that elucidation of the details of these events at the structural level in cells which can be grown under chemically defined conditions and induced to interact in several different ways, without resort to virus treatment or chemical procedures, will increase our understanding of neoplastic cell growth and malignant transformation in mammalian cells. Thin sectioning, ultrastructural-cytochemical, and freeze-etch preparation techniques will be used to examine the structural bases of contact inhibition, the effects of culture conditions and ploidy on membrane intercalated particles, and to test these hypotheses regarding cell fusion: Cell fusion occurs because of: A lysosome caused loss of the cell coat. (2) Addition of new plasmalemma lacking cell coat. (3) Change in composition of polysaccharide in cell coat. (4) Major changes in proteins of cell membrane. Specific approaches will include use of stains specific for cell coat polysaccharides, experiments with lectins, immunocytochemistry, and cytochemical localization of lysosomal enzymes.